This invention relates generally to the field of desiccation and more specifically to a process for the removal of water from evacuated chambers or from gases by means of boron oxide obtained by boric acid decomposition under vacuum or under a dry gas flow.
Water is one of the main contaminants in vacuum systems and in gases for advanced applications, such as those used in the semiconductor industry. Consequently, numerous industrial applications call for the removal of water and water vapor. Water vapor needs to be removed from evacuated spaces employed as thermal insulation, such as the vitreous or metallic gaps in thermos flasks and the evacuated panels filled with polymeric materials used in refrigeration systems. The use of gas sorbing materials inside such panels is disclosed, for example, in U.S. Pat. No. 5,544,490. Another application for water sorption includes the manufacture of mechanical microdevices, sometimes referred to as micromachines or MEMs. A further example of the need to remove water is in polymer-encapsulated integrated circuits as described in U.S. Pat. No. 4,768,081.
Another important application is in laser devices, for example power lasers used in amplifiers for optical fiber communications (hereinafter referred to as "optical amplifiers"). Optical amplifiers consist, in large part, of a lasing source in an enclosed chamber filled with an inert gas, typically nitrogen. Upon their manufacture, optical amplifier chambers frequently contain hydrocarbon impurities as a result of the production process. These impurities tend, over time, to lower the efficiency of the device by forming an obscuring deposit on the laser's exit window. In order to eliminate these impurities, small amounts of oxygen are added to the nitrogen atmosphere. The laser beam causes the oxygen to react with the hydrocarbons to form water and CO.sub.2. The CO.sub.2 does not interfere with the act or operation of the optical amplifier, however the water has to be removed. The use of impurity getters in laser enclosures is disclosed in European Patent Application EP707360 A1 published Apr. 17, 1996 and issued as EP707360 B1 on Mar. 4, 1998.
Water removal is also extremely important for purified gases, especially as used in the microelectronics industry for deposition and etching act or operations. The purity levels needed for these process gases continue to increase as the tolerances for defects continue to decrease. For example, the industry currently requires noble gases such as helium and argon to contain no more than about 5 parts per billion (ppb) of total impurities. The presence of water vapor is particularly serious in halogen and halogenated gases such as chlorine, hydrogen fluoride, hydrogen chloride, hydrogen bromide, silicon tetrachloride, trichlorosilane and dichlorosilane. Traces of water in these gases, widely used in microelectronics industry, form highly corrosive compounds such as hydrofluoric acid inside gas pipelines and reaction chambers. Corrosive processes create particles in these ultraclean environments lowering yields and necessitating costly downtime and equipment replacement. Other gases employed in the industry, from which water needs to be eliminated, include, among others, boron compounds such as boron trichloride, boron trifluoride, and diborane; nitrogen compounds such as nitrogen trifluoride, nitrous oxide, nitric oxide, and nitrogen dioxide; hydrides such as silane, arsine, phosphine; sulfur hexafluoride and tungsten hexafluoride; chlorine trifluoride; hydrazine and dimethyl hydrazine.
Water removal from vacuum chambers and process gases typically is carried out by means of chemical or physical sorbents. Examples of physical sorbents include zeolites, porous alumina, and silica gel. These sorbents are not suitable for many high technology applications, however, because their sorption of water, as well as of the other gases, is reversible, and the sorbed gases may be released in the presence of a high vacuum or upon heating. Another problem occurs when the process gas itself is sorbed, for example, when certain zeolites are used to remove water vapor from gaseous HCl, as sorbed HCl diminishes the sorption efficiency for water.
Chemical sorbents have been known for a long time. The most effective chemical sorbents have been found to be alkaline-earth metal oxides, particularly barium and calcium oxides, and perchlorates of magnesium and barium. Other strong chemical sorbents include copper sulfate, calcium and zinc chlorides, and phosphorus pentoxide. Some of these materials, however, are not suitable in particular applications. For example, alkaline-earth metal oxides are basic and cannot be used for removing water from halogen or halogenated gases because they chemically react with these gases.
A third class of materials suitable for chemical moisture sorption include zirconium- and titanium-based alloys, generally known as non-evaporable getter alloys. These alloys sorb a wide range of gases, including O.sub.2, CO, CO.sub.2, and water. Unfortunately, the sorbing capacity of these alloys at room temperature is very limited. Additionally, these materials cannot be used to purify reactive gases, such as the above-mentioned halogen and halogenated gases, as they react with these gases to form metal halides which then contaminate the process gas.
The problem of water removal from halogen or halogenated gases has prompted the development of new materials. U.S. Pat. Nos. 4,853,148 and 4,925,646 disclose water removal from HF, HCl, HBr and HI by means of supported metal halides having the general formula MX.sub.y, where X is a halogen element and y corresponds to the valency of the metal M, which may be 1, 2, or 3. Additionally, these patents disclose metal halides of the form MX.sub.y-1 that may be covalently bonded to a support. U.S. Pat. No. 4,867,960 discloses the use of SiCl.sub.4 and chlorides of metals with valencies of at least four, with or without support, for water removal from HCl. Finally, U.S. Pat. No. 5,057,242 discloses the removal of water from chlorosilane gases by using materials of the general formula R.sub.a-x MCl.sub.x, where R is an alkyl, x is in the range of 0 to a, and M is a metal selected from the group consisting of the alkali metals, alkaline-earth metals, and aluminum.